1. Field of the Invention
The present invention relates to a new-type cubic boron nitride and its gas phase synthesis method.
2. Discussion of Background
Cubic boron nitride is expected to be a potentially useful functional material in view of its characteristics such that it has the highest hardness and thermal conductivity next to diamond, it has the low reactivity with iron-family metals, and it has a wide band gap and can be a semiconductor. Its crystal grains have already been synthesized industrially under a high pressure of tens of thousands atom by using a super high pressure apparatus. However, with this method, it is difficult to control its form, and particularly it is very difficult to synthesize cubic boron nitride in the form of a film.
On the other hand, attempts have been done to synthesize it from a gas phase under a pressure lower than 1 atom. These methods are classified into two groups. One is physical vapor deposition methods (PVD) by utilizing ion bombardment to a substrate, including gas-activated reactive evaporation, bias-sputtering and bias ECR. In these methods, the pressure in a reaction chamber is as low as at most 10xe2x88x921 Torr.
The other is chemical vapor deposition methods (CVD method), wherein a boron source such as diborane, boron trichloride, boron trifluoride, aminoborane or borazine, and a nitrogen source such as ammonia or nitrogen, are reacted. To suppress formation of hexagonal boron nitride, atomic hydrogen generated by a plasma or a heated filament, is utilized in many cases.
In the above CVD methods, the pressure in the reaction chamber is usually so high as from 10 to 160 Torr except for some cases. The CVD method without substrate-biasing is unestablished since a follow-up experiment has not succeeded. By CVD methods with substrate-biasing under a pressure of from 10xe2x88x923 to 10xe2x88x921 Torr, synthesis of cubic boron nitride has been reported.
However, cubic boron nitrides obtained in any of the above methods are usually a mixture with e.g. hexagonal system, or have poor crystal quality, and X-ray diffraction peaks of cubic boron nitride are not detected or are very broad, although an absorption can be seen in the vicinity of from 1050 to 1100 cmxe2x88x921 in the infrared absorption spectrum. In the infrared spectroscopic analysis, the absorption peak of cubic boron nitride is seen in the vicinity of from 1050 to 1100 cmxe2x88x921, and the absorption peak of hexagonal-type boron nitrides, such as hexagonal boron nitride, turbostratic boron nitride or amorphous boron nitride is seen in the vicinity of from 1360 to 1400 cmxe2x88x921 and in the vicinity of 800 cmxe2x88x921, and it is considered that the ratio of cubic boron nitride and hexagonal-type boron nitrides corresponds to the ratio of the absorbance of the peak at from 1050 to 1100 cmxe2x88x921 to the absorbance of the peak at from 1360 to 1400 cmxe2x88x921.
On the other hand, it is said that cubic boron nitride synthesized by high pressure method shows a Raman scattering peak by an optical longitudinal wave mode phonon at about 1305 cmxe2x88x921 and a peak by an optical transverse wave mode phonon at about 1056 cmxe2x88x921, and the intensity ratio of these two changes by the relation between the optical axis of the excitation light and the crystal axis. The peak position of the Raman scattering changes by e.g. stress applied to the crystal, crystal defects and the size of the crystal. It reversely varies by compression stress and tensile stress, and in the case of a film obtained from a gas phase, the stress is at most 10 GPa, and the variation range due to the stress is 10 cmxe2x88x921. However, when the effect of crystal defects and crystal size are added, the variation range becomes 15 cmxe2x88x921. Further, the full width at half maximum (FWHM) of the Raman scattering peak is about 5 cmxe2x88x921 for a single crystal of cubic boron nitride having a high quality and a size of from 1 to 2 mm obtained by a high pressure method, and it becomes broader if the crystal size becomes small or crystal defects increase. In the case of a cubic boron nitride film obtained by the conventional gas phase synthesis, the crystal size is several nm, and the Raman peaks has not been observed. Here, the crystal size was estimated by FWHM of the X-ray diffraction peaks or by electron microscope observation, and the FWHM of 111 reflection by CuKxcex1 ray is about 3xc2x0 in 2xcex8 when the crystal size is 5 nm, and the higher the index, the broader. The appearance of the Raman peaks indicates high crystal quality. Gas phase synthesis of cubic boron nitride distinctly showing these scattering peaks and proving to be truly cubic boron nitride by other charaterization methods also, has not been reported.
By known literatures, S. Manorama et al, J. Phys. D: Appl. Phys. 26(1993) 1793; K. Liao and W. Wang, phys. stat. sol. (a) 145(1994) K25; K. Liao and W. Wang, phys. stat. sol. (a) 151(1995) K1; Y. Guo et al, phys. stat. sol. (a) 143(1994) K13; F. Zhang et al, Appl. Phys. Lett. 65(1994) 971 and z. Song et al, Appl. Phys. Lett. 65(1994) 2669, with respect to the synthesis of cubic boron nitride, success in follow-up experiments has not been reported, and the success is very questionable. Accordingly, it can be stated that synthesis of cubic boron nitride of high quality presenting the Raman peaks as mentioned above has not been successfully carried out by the conventional low-pressure synthesis methods.
It has been known that confirmation of cubic boron nitride has to be carried out by at least three conventional methods including X-ray diffraction, infrared spectroscopy and compositional analysis, and the Raman spectroscopy is required to confirm cubic boron nitride of high quality.
The present inventors have invented a method for synthesizing high-density boron nitride by means of plasma CVD method employing boron trifluoride and a nitrogen-containing gas as starting materials before (JP-A-8-208207). However, it was not cubic boron nitride, but turbostratic boron nitride, in view of a crystal structure.
Under these circumstances, the present invention has been made to overcome the above problems, and to provide a method for synthesizing cubic boron nitride having high crystal quality from a low-pressure gas phase.
The present inventors have found that cubic boron nitride having good crystal quality can be obtained by using a fluorine or a fluorine-containing gas species as one component of starting materials, and by using substrate-biasing, and the present invention has been accomplished.